This invention relates to a flame-retarded copper-clad laminate. More particularly, it relates to such laminates made of paper substrate and unsaturated polyester resin having a copper cladding on one or both sides thereof.
Flame-retarded copper-clad laminates are widely used for safety reason in many electrical and electronic devices as printed circuit wiring boards. They are usually composed of a plurality of substrate layers each impregnated with a resin and a copper cladding foil bonded onto one or both sides thereof. However, attempts have never been made, to our best knowledge, to produce a flame-retarded copper-clad laminate from paper substrates and unsaturated polyester resins.
Only those flame-retarded, copper clad unsaturated polyester resin laminates in which a nonflammable substrate such as glass cloth is used have so far been commercialized in the United States and some other countries. In cases where the substrate is nonflammable, it is easy to select an adequate, flame-retarded, unsaturated polyester resin, since said resin has only to be flame-retarded by itself. Thus, in most cases, reactive flame retardants containing aromatic bromine, such as tetrabromophthalic anhydride, are used as a saturated dibasic acid component for the production of polyesters.
However, when a resin of that kind is used in producing paper-based copper-clad laminates, the laminates can by no means be self-extinguishing because the paper substrate is inflammable. Therefore, it becomes necessary to increase the bromine content by increasing the amount of said reactive flame retardants and/or by adding non-reactive aromatic bromine compounds.
Investigations made by the present inventors have revealed that, for obtaining flame-retarded paper/polyester copper-clad laminates which can meet the requirements under UL-94 VO, for instance, the aromatic bromine content in the unsaturated polyester resin should be at least 30 percent by weight. The mechanical strength and copper foil peel strength of the laminate obtained were only slightly deteriorated by heating, for example at 100.degree. C. to 170.degree. C.
However, aromatic bromine contents exceeding 30 percent by weight not only increase the cost of production of the laminate but also unfavorably cause rapid decrease in tracking resistance. It is well known that halogen compounds such as bromine compounds and chlorine compounds, phosphorus compounds, antimony trioxide and the like may be used as flame retardants for unsaturated polyester resins. Among them, bromine compounds are most effective.
Generally, flame retardant bromine compounds may be classified roughly into the following five groups:
(1) Aromatic bromine compounds having, for example, by the formula: ##STR1## wherein R, and R' each means a hydrocarbon substituent;
(2) Aliphatic bromine compounds represented, for example, by the formula: EQU R--CH.sub.2 Br
wherein the bromine is bonded to a primary carbon atom;
(3) Aliphatic bromine compounds represented, for example, by the formula: EQU R--CHBr--R'
wherein the bromine atom is bonded to a secondary carbon atom;
(4) Aliphatic bromine compounds represented, for example, by the formula: ##STR2## wherein the bromine atom is bonded to a tertiary carbon atom; and
(5) Alicyclic bromine compounds represented, for example, by the formula: ##STR3## When evaluated as flame retardants, compounds (2)-(5) are more effective than compounds (1); compounds (3) are more effective than compounds (2); compounds (4) are more effective than compounds (3); and compounds (3) and compounds (5) are comparable to each other.
Attempts have been made by us to use compounds (5) as flame retardant for paper/polyester copper-clad laminates. As a result, it has been found that, at a bromine content in the resin of 20 percent by weight or less, the resultant laminates meet the requirements of the UL standard UL-94 VO. Regretably, however, whereas those laminates were excellent in fire resistance and tracking resistance, heating at high temperatures, for example at 100.degree.-170.degree. C., resulted in marked deterioration of mechanical strength, discoloration of copper foil, and/or remarkable decrease in copper foil peel strength. Said laminates were thus inadequate with respect to heat stability.